Killer cave lured ancient carnivores to their deaths

A cavern in Spain may have lured ancient carnivores to their deaths by offering the promise of food and water, new research suggests.

The new study, published May 1 in the journal PLOS ONE, may explain how the carcasses of several carnivore species, including saber-toothed cats and "bear dogs," wound up in an underground cavern millions of years ago.

In 1991, miners drilling about 18.7 miles (30 kilometers) outside Madrid, Spain, uncovered animal bones and called in local paleontologists to excavate. They uncovered a series of underground caverns full of animal fossils — including the remains of red pandas, bear dogs and saber-toothed cats, as well as ancient animals related to modern elephants, giraffes, rhinoceroses and horses. To date, nearly 18,000 fossils have been recovered from the area.

The so-called Cerro de los Batallones caves formed between 9 million and 10 million years ago in a process known as piping, where sediment falls into cracks or fissures in the surface, carving out hollow spaces in the soft, claylike Earth. Unlike other "pipes" formed in clay that quickly disappear, however, the holes stayed open longer because the area also contained a harder type of rock to buttress the burgeoning caves.

Exactly what happened to the animals, however, remained a mystery.

Cave of deathTo solve the mystery, Domingo and her colleagues systematically analyzed all the bones discovered in the bottom level of the site, called Batallones-1. The cave contained an unusually high proportion of carnivores, some of which were completely unknown before the cave's discovery, Domingo told LiveScience.

The researchers found that nearly 98 percent of the large-mammal fossils were of carnivores, many of them healthy young adults. They also studied the orientation of the skeletons in the cave and the lack of trample marks.

Based on these findings, the team concluded that the cave probably had a visible opening in the ground, into which the herbivores may have occasionally fallen. (For instance, the cave contains a complete rhinoceros skeleton.) However, most of the herbivores were smart enough to avoid the trap.

But young-adult carnivores willingly entered the cave, possibly lured by the scent of food or water. The unlucky animals became trapped and died (likely from starvation), and the scent of their rotting carcasses lured other carnivores into the caves, and to their deaths.

"We think that the carnivores were getting trapped, and then they became additional food for another coming carnivore," Domingo said.

The researchers still don't know how long the cave was open before flooding brought fresh material to cover it up. And the findings only apply to one cavern at the site — scientists still need to do further studies to determine how herbivores wound up in the upper levels of the cave complex.